Electrographic imaging system and heads therefor

ABSTRACT

An electrographic imaging system is provided for producing an electrostatic latent image upon an electrographic record medium having dielectric and conductive layers. Electrostatic charging means are provided comprising an array of charging electrode means, some of which are interconnected and thereby are connected to fewer first voltage source means than the total number of charging electrode means in the array. Also provided is an array of complementary electrode means which is connected to second voltage source means. The energized complementary electrode means are capacitively coupled to the conductive layer of the record medium to change the potential thereof upon a voltage pulse being applied to the complementary electrode means. The energized charging electrode means charge discrete areas of the dielectric layer of the record medium only when the complementary electrode means simultaneously are connected to the second source of voltage to effect the capacitive coupling. Also, heads are provided wherein an array of small electrical conductors is supported in a dielectric material in closely spaced relation to each other with the end faces thereof exposed and substantially flush with the support. Complementary electrode means are mounted on the support adjacent to but spaced from the end faces of the electrical conductors and form with the support means a substantially flush, smooth surface over which the record medium can move. Electrical Contacts for both the small electrical conductors and the complementary electrodes are provided in the support.

United States Patent [72] Inventor John Blumenthal Wickliffe, Ohio [211App]. No. 812,821

[22] Filed Apr. 2, 1969 [45] Patented Oct. 5, 1971 [73] Assignee CleviteCorporation Continuation-impart of application Ser. No. 37,210, May 14,1970.

[54] ELECTROGRAPHIC IMAGING SYSTEM AND HEADS THEREFOR 12 Claims, 10Drawing Figs.

[52] U.S.Cl 346/74ES [51] Int. Cl Gold 15/06 [50] Field of Search 346/74ES,

74 E, 74 s, 74 SB; l0l/D1G. 13; 250/495 [56] References Cited UNITEDSTATES PATENTS 2,997,361 8/1961 Christopherson et al..... 346/742,898,468 8/1959 McNaney 346/74 X 3,112,693 12/1963 Williams 346/74 X3,178,718 4/1965 Baron 346/74 3,438,053 4/1969 Howell 346/74 3,470,5639/1969 Starr 346/74 Primary Examiner-Bernard Konick AssistantExaminer-Gary M. Hoffman Attorney-Ebcr J. Hyde ABSTRACT: Anelectrographic imaging system is provided for producing an electrostaticlatent image upon an electrographic record medium having dielectric andconductive layers. Electrostatic charging means are provided comprisingan array of charging electrode means, some of which are interconnectedand thereby are connected to fewer first voltage source means than thetotal number of charging electrode means in the array. Also provided isan array of complementary electrode means which is connected to secondvoltage source means. The energized complementary electrode means arecapacitively coupled to the conductive layer of the record medium tochange the potential thereof upon a voltage pulse being applied to thecomplementary electrode means. The energized charging electrode meanscharge discrete areas of the dielectric layer of the record medium onlywhen the complementary electrode means simultaneously are connected tothe second source of voltage to effect the capacitive coupling. Also,heads are provided wherein an array of small electrical conductors issupported in a dielectric material in closely spaced relation to eachother with the end faces thereof exposed and substantially flush withthe support. Complementary electrode means are mounted on the supportadjacent to but spaced from the end faces of the electrical conductorsand form with the support means a substantially flush, smooth surfaceover which the record medium can move. Electrical Contacts for both thesmall electrical conductors and the complementary electrodes areprovided in the support.

SHEET 2 UF 3 INVENTOR.

JOHN BLUMENTHAL BY v ATTORNEY PATENTED 0m 5 I97! SHEET 3 OF 3 ATTORNEYELECTROGRAPI-IIC IMAGING SYSTEM AND HEADS THEREFOR A continuation-impartapplication Ser. No. 37,210 was filed May I4, 1970, and assigned to thesame assignee as the present application.

BACKGROUND OF THE INVENTION l Field of the Invention This inventionrelates to electrographic imaging systems and heads and, moreparticularly, to an electrostatic charging means for formingelectrostatic latent images on a record medium in accordance withinformation received from external electrical signals.

The electrographic recording process is generally characterized by twobasic steps. The first step is the establishment of an electrostaticlatent image by electrically charging areas on predetermined portions ofa record medium by electrostatic recording heads connected to chargingcircuit means. In the second step the electrostatic latent image isrendered visible by the toning or developing of the charged areas on therecord medium.

The present invention is directed to an improvement in the first step,that is, an improved electrographic imaging system for the establishmentof the electrically charged areas on the record medium, and to improvedheads for charging the record medium. The present invention achieves itshighest degree of utility in a system which employs fewer voltage supplycircuit means than the total number of charging electrodes.

2. Description of the Prior Art In general, the prior art providescharging electrodes which terminate adjacent a dielectric layer of therecord medium upon which it is desired to deposit an electrostaticcharge. A backing bar or electrode of electrically conducting materialis located opposite the side of the record medium on which theelectrostatic charge is to be deposited. For example, in U.S. Pat. No.2,9l9,l7l, issued Dec. 29, 1959, to Epstein and Phelps, the recordmedium is threaded between the charging electrodes and the backing baror electrode and is held in intimate contact with the latter whileremaining spaced from the former. The signal information voltage ofgiven polarity is applied to the charging electrodes, and it is notgreat enough, by itself, to charge an area of the record medium. Asupplemental voltage of opposite polarity and of a magnitudeinsufficient by itself to cause charging of the record medium is appliedby physical contact to the backing bar. When the two voltages aresimultaneously applied, the resultant total voltage is sufiicient tocause an electrical charge to be applied from the charging electrodeacross the airgap to the surface of the dielectric layer.

An object of the invention is to provide an improved electrostaticrecording head and recording system wherein capacitive coupling isutilized, thereby eliminating the necessity of making good physicalcontact between the head and the record medium.

Another object of the present invention is to provide an electrostaticcharging head which can be located only to one side of the record mediumthereby eliminating backing electrodes and providing easier threading ofthe record medium across the electrostatic charging means.

A further object of the present invention is to provide anelectrographic imaging system that because of capacitive coupling isessentially insensitive to the thickness and variation in thickness ofthe record medium.

A further object of the present invention is to eliminate the necessityof maintaining alignment between the charging electrodes and the backingelectrodes.

A still further object of the present invention is to reduce the hazardof accidental electrical shorting between a backing electrode and acharging electrode.

Further objects and advantages of the invention will appear from thefollowing detailed description to be read in conjunction with theaccompanying drawings wherein like reference symbols denotecorresponding parts throughout the several views.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided an electrographic imaging system for use with a recordmedium having a dielectric layer with an exposed surface and having aconductive layer in contact with the opposite face of the dielectriclayer. Charging electrode means having end face means are mountedadjacent to the surface of the dielectric layer, and complementaryelectrode means are mounted adjacent to the record medium and in closelyspaced proximity to the charging electrode means. Voltage less than thecritical value required for imaging is applied to the charging electrodefrom first voltage means. A pulse of voltage less than the criticalvalue is applied to the complementary electrode means from a secondvoltage source. The complementary electrode means are capacitivelycoupled to the conductive layer of the record medium. The voltage pulseby virtue of capacitive coupling changes the potential of the conductivelayer directly adjacent the charging electrode means in coincidence withthe voltage applied to the charging electrode means to fonn on therecord member an electrostatic latent image of the end face means of thecharging electrode.

Further, in accordance with the invention, there are providedelectrostatic charging heads for use with the aforedescribed recordmedium, wherein an array of small electrical conductors is supported ina dielectric material in closely spaced relation to each other with theend faces thereof exposed and substantially flush with the support.Complementary electrode means are mounted on the support adjacent to butspaced from the end faces of the electrical con ductors and form withthe support means a substantially flush, smooth surface over which therecord medium can move. Electrical contacts for both the smallelectrical conductors and the complementary electrodes are provided inthe support.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of a portion of an electrostatic chargingmeans or head adjacent to the exposed surface of a dielectric layer ofan electrographic record medium on which an electrostatic latent imageis to be formed; the head being a single-row aligned head. FIG. 2 is adiagrammatic representation of a portion of an electrostatic imagingsystem, and of a head with portions removed, adjacent to theelectrographic record medium.

FIG. 3 is a cross-sectional view along line 3-3 in FIG. 2, illustratingthe relationship of the charging electrode means and the complementaryelectrode means to the record medium.

FIG. 4 is a diagrammatic representation of one charging electrode meansand an adjacent complementary electrode means in conjunction with theelectrographic record medium.

FIG. 5 is a simplified equivalent circuit representing the components asshown in FIG. 4 and having the equivalent circuit elements arranged inparallel position to these components.

FIG. 6 shows schematically the arrangement of larger charging electrodesin a two-row offset head for denser recording.

FIG. 7 shows schematically the arrangement of still larger chargingelectrodes in a three-row offset head for denser recording.

FIG. 8 schematically shows rectangular charging electrodes in a charginghead for denser recording.

FIG. 9 shows angularly related single-row charging heads for denserrecording than is obtained from the head shown in FIG. 1.

FIG. 10 schematically shows rectangular charging electrodes arranged ingroups wherein each group is capable of producing alpha-numericcharacters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of thedrawing, there is shown an electrographic imaging system and headembodying the present invention. An electrostatic charging means or head12 is shown with an electrographic record medium 14 which is givenrelative displacement to the charging means 12 by means not shown.

The electrographic record medium 14 comprises a dielectric layer orstratum 16 in contact with a conductive layer or stratum 18. Thedielectric layer 16 has an exposed charge retentive surface 17 which, inoperation, substantially engages the surface of the charging means 12.Alternatively, the record medium may comprise a conductive layer betweentwo layers, the top layer being less conductive than the middle layer.

The charging means or head 12 comprises a linear array of closely spacedcharging electrode means 20 having relatively small-area exposedportions or ends 21, and complementary electrode means 22 havingrelatively large-area exposed portions 23.

The charging electrode means 20 are generally small, closely spaced,electrical conductors embedded in a support 24 composed of a suitabledielectric material such as a plastic or a ceramic, with their end faces21 substantially flush with the end surface of support 24. By way ofillustration, the charging electrode means 20 can be approximately milsin diameter and spaced on approximately l2.5-mil centers so that theyare separated by about 2.5 mils. The charging electrode means shown inFIG. I are arranged in a linear array; however, they are adaptable to avariety of nonlinear arrangements and may represent alpha-numericsymbols or the like.

In operation, it is required that the capacitance of the complementaryelectrode means 22 to the conductive layer 18 of the electrographicmedium 14 be large as compared to the capacitance of the chargingelectrode means 20 to said conductive layer. The capacitance between thecomplementary electrode means 22 and the conductive layer is preferably10 or more times the capacitance of the sum of the charging electrodemeans 20 to the conductive layer. Since capacitance is directlyproportional to the physical area of the plates of a capacitor, theexposed surface portion means 23 of a complementary electrode means 22preferably is at least 10 times that of the sum of the exposed endportion means 21 of an associated group of charging electrode means 20.

The complementary electrode means 22 are mounted on the support 24 sothat their surfaces 23 are substantially flush with the end surface ofthe support 24 and the end faces 21 of the charging electrodes 20,forming a smooth, preferably slightly curved surface which the recordmedium engages during operation of the system. The complementaryelectrode means 22 are generally rectangularly shaped electricalconductors having their long sides in a parallel relationship to eachother, and having the charging electrode means 20 centered between andparallel to two complementary electrode means 22. While the exposedportion means 23 of the complementary electrode means 22 are shown aselectrical conductors they can include a dielectric coating 23' (seeFIG. 4). In such a case the thickness of the dielectric coating 23' isdependent on the dielectric coefficient of the material. An example of adielectric material which has been used successfully is barium titanate.The coating 23' provides complete protection against accidentalshorting.

The charging electrode means 20 extend down into the support 24 and bymeans of conductors 26 are connected first voltage circuit means 42(FIG. 2). Conductors 30 are internally connected to the complementaryelectrodes 22 and extend through the support 24 to complementary voltagecircuit means 40. The voltage circuit means 40 and 42 receive electricalsignals from logic circuits adaptable to the particular application forthe graphic display unit such as computer printout, data-plotter,oscillograph, etc.

The area of the exposed portion means 23 of each complementaryelectrodemeans 22, as shown in the present embodiment, is substantially greaterthan the area of the exposed portion means 21 of the charging electrodemeans 20 flanked by the complementary electrode means. This is inaccordance with the area requirements which are essential to provide theproper capacitive relationships in this invention. By way ofillustration, the exposed portion means 23 may be approximately 3l2 milslong and approximately 200 mils wide. Thus, 25 charging electrode means,each about 10 mils in diameter, would have a total area of about 2,000square mils and the two complementary electrode means 22 would have anarea of about 125,00 square mils.

OPERATION OF THE ELECTROSTATIC CHARGING MEANS As the record medium 14moves across the electrographic charging means 12, the chargingelectrode means 20 and the complementary electrode means 22 are in closeproximity to one side of the record medium 14. Preferably, the exposedsurface 17 of the record medium 14 is spaced at a given very shortdistance from the exposed portion means 21 of the charging electrodemeans 20 to regularize the discharge from the charging electrode means.This can be accomplished by using a record medium that assures a minimumspacing between the electrode means and the record medium by the recordmedium itself, as provided in copending U.S. Pat. application Ser. No.694,654, filed Dec. 29, 1967, in the names of Arling Dix Brown, Jr., andJohn Blumenthal, and assigned to the same assignee as the presentinvention.

First voltage means 42 are electrically connected by conductors 26 and52 to the charging electrode means 20 and 20' for applying thereto, whenswitch 43 is closed, a first voltage of given polarity and of a firstpredetermined amplitude.

Second voltage means 40 are electrically connected to the complementaryelectrode means 22 by conductors 30 and 58, when switch 41 is closed,for applying thereto a voltage pulse of polarity opposite to theaforesaid given polarity, and of a second predetermined amplitude. Thisvoltage pulse must have a duration sufficiently long to change bycapacitive coupling the potential of the conductive'layer 18 under thecharging electrode means. This change of potential in coincidence withthe presence of the first voltage at the charging electrode means 20causes an electrostatic latent image of the exposed end 21 of thecharging electrode 20 to be formed on the record medium at a locationopposite the charging electrode means. It is only when voltage isapplied to charging electrode means 20 in coincidence with the properpotential in the conductive layer 18 established by capacitive couplingwith the energized complementary electrodes that an electrostatic imagewill be formed. If either condition is absent, no image will be formed.

The adjacent pair of complementary electrode means 22' (that are notshaded) are not energized as switch 41' at this instant is connected toground. Charging electrode means 20' are connected by line 52 to voltagesource 42 and are therefore energized through closed switch 43, butthere is no electrification of the charge retentive surface 17 byelectrode means 20' because there is no simultaneous supplementalvoltage from the adjacent complementary electrode means 22. To causecharging electrode 20' to produce an electrostatic image the energizingof complementary electrodes 22' adjacent to it would be required. Thisis shown, described. and claimed in U.S. Pat. application Ser. No.868,394 filed Oct. 22, 1969, in the name of Arling Dix Brown for anElectrographic Recording System, and assigned to the same assignee asthe present application.

In the present embodiment there are two rows of flanking complementaryelectrode means. The system of the present invention will also work byusing a single row of complementary electrode means, but usually with anattending degradation in performance stemming from less effectivecapacitive coupling of the opposite polarity voltage pulse into theregion of the charging electrode means with which it is desired to makean electrostatic latent image on the surface of the record medium.

Referring to FIG. 3, an enlarged cross-sectional view of a chargingelectrode means 20 and a pair of complementary electrode means 22 areshown to illustrate the capacitive coupling between the electrostaticcharging means and the record medium. When the second voltage fromvoltage means 40 is applied to the complementary electrode means 22, thecapacitive coupling to the portions of the conductive layer 18 withinthe brackets 44 cause these area to rise in potential. The portion ofthe conductive layer 18 indicated by bracket 46 must reach a voltagesufficiently high that when the first voltage from means 42 is appliedto the charging electrode means 20, the total potential differenceacross the gap 50 is equal to or greater than the critical chargingpotential. This causes an electrostatic latent image of the electrodemeans 20 to be formed on the surface 17 of the dielectric layer 16 at alocation directly under the charging electrode means 20.

EFFECT OF THE CHARACTERISTICS OF THE RECORD MEDIUM The followingdiscussion is based on the simplified equivalent circuit, FIG. 5, usinglumped values to approximate the actual distributed resistances andcapacitances.

Considered in this equivalent circuit are the effects produced by onecharging electrode means 20 and an adjacent complementary electrodemeans 22 in conjunction with the electrographic record medium 14 asshown in FIG. 4. First voltage means V, 60 represent signal voltageapplied to the charging electrode means 20 which preferably are ofnegative polarity with respect to the conductive stratum 18 of therecord medium. Second voltage means V, 72 represent the voltage pulseapplied to the complementary electrode means 22 and which is generally apulse of similar magnitude but of opposite polarity to the voltageapplied to the charging electrode means 20. Capacitor 62 represents thecapacitance from the exposed portion means 21 of the charging electrodemeans 20 to the adjacent charge retentive surface 17 of the dielectriclayer 16 of the record medium 14. Capacitor 64 represents thecapacitance between the exposed charge retentive surface 17 of thedielectric layer 16 and the conductive layer 18 of the record medium 14,under the charging electrode means 20. Since capacitors 62 and 64 can beconsidered as a voltage divider, it becomes desirable that thecapacitance of capacitor 62 be small compared to the capacitance ofcapacitor 64. The plate separation for both capacitors being essentiallyidentical in most practical instances, flexibility of choice ofdielectric material used for the coating 17 on the medium provides somemeasure of control of this ratio.

Resistor 66 represents the resistance of the conductive layer 18 of therecord medium 14 along the path linking the area adjacent thecomplementary electrode means 22 and the are under the chargingelectrode means 20. The effective value of resistor 66 is determined bythe geometry of the electrostatic charging means and the sheetresistance of the conductive layer 18 of the record medium 14. Theresistor 66 coacts primarily with the capacitors 62, 64, and 68 todetermine the rise time of the voltage across the gap, i.e., acrosscapacitor 62. Thus, if the value of resistor 66, as determined by thegeometry of the electrostatic charging means and the sheet resistance ofthe conductive layer, is very large, the rise time will be increased andmay not allow critical charging potential to be attained within therequired time.

Capacitor 68 represents the capacitance between the electrode means 22and the conductive layer 18 of the record medium 14. Capacitor 68 has ahigh capacitance compared to the capacitance of capacitors 62 and 64 inseries, e.g., to 50 times greater.

Resistor 70 represents the resistance of the conductive layer 18 fromthe area to be energized to ground. The resistance of resistor 70 isdetermined by sheet conductance of the conductive layer of the recordmedium and by grounding geometry.

The resistor 70 coacts primarily with capacitor 68 in determining thedecay time of the voltage coupled into the energized area. For properoperation, the gap voltage (the voltage across capacitor 62) must reachcritical value when v: is applied during the presence of V,

In a functional system, where pulses as short as 1.0 microsecond wereemployed, it was discovered that satisfactory performance could beobtained using record media with conductive layers having sheetresistances ranging from 0.25 to 1.0 megohm/sq. Suitable thicknesses forthe dielectric layer range from 0.1 to 0.25 mil. Using the embodimentheretofore described, and utilizing the invention of U.S. Pat.application Ser. No. 694,654, it has been found that the total voltagerequired for obtaining the critical charging potential between thecharging electrode means and the record medium is approximately 500volts.

FIG. 1 shows a head with a single line of charging electrodes 20extending at an angle of to the direction of motion of the recordmember. If the electrodes are 10 mils in diameter and are spaced 2.5mils apart, they are capable of drawing a latent image across the recordmedium which, when toned, would produce a dark grey line due to thehalftone effect. In the straight-line head the maximum electrodediameter is approximately the electrode spacing. In order to produce acontinuous black line, different head and electrode arrangements arenecessary.

FIGS. 6, 7, and 8 show alternate arrangements for the chargingelectrodes of a head.

In FIG. 6 there is shown two spaced-apart rows of electrodes 20A and20B, and the arrow indicates the direction of motion of the recordmedium in respect thereto. This arrangement is particularly useful wheremore solid black is required. and where larger diameter chargingelectrodes are wanted. The electrodes 208 should be staggered in respectto the electrodes 20A. Overlapping imprints can be obtained by advancingthe paper. In this arrangement the diameter of the electrode can belarger than the nominal electrode spacing which is not true of thestraight-line head.

FIG. 7 shows an arrangement wherein the charging electrodes are arrangedin three spaced-apart, staggered rows 20C, -D, -E. This configuration isuseful when still larger diameter electrodes are used and, as shown, theleft-hand side of the electrodes 20D overlap the right-hand side of theelectrodes 20C by an amount equal to one-half diameter, and theleft-hand side of electrodes 20E overlap the right-hand side ofelectrodes 20D by one-half diameter, and the right-hand side ofelectrodes 20E overlap the left-hand side of electrodes 20C by one-halfdiameter. This will give good resolution at higher image density. Withthis arrangement the electrode diameter can be larger than two times thenominal electrode spacing; i.e., up to approximately three times thenominal spacing indicated by the marks.

It is not essential that the charging electrodes be round in crosssection. FIG. 8 shows rectangular charging electrodes 20F in two spaced,staggered rows. between complementary electrodes 22. This arrangement issimilar to the arrangement of FIG. 6 in respect to producing asubstantial solid line after toning. Similar to FIG. 7 the rectangularelectrodes can overlap each other.

FIG. 10 shows the rectangular charging electrodes 20F may be arranged ingroups, matrixlike, so that by selection of the proper electrodes,letters and numerals may be obtained.

FIG. 9 indicates an arrangement wherein a plurality of the recordingheads 12, shown in FIG. 1, are mounted with the electrode line at anangle other than 90 to the direction of motion of the record member. Forexample, as shown, the heads 12 are at 45 to the direction of motion. Byproperly relating the charging electrode diameter, the spacing betweenthe electrodes, and the head angle, the system can produce essentiallysolid lines giving records with good contrast.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What 1 claim'as new and desire to secure by Letters Pat. Of the US. is:

1. In an electrographic imaging system comprising a record medium havinga conductive layer in contact with a dielectric layer with an exposedsurface, and an electrostatic charging means including:

charging electrode means having end face means adjacent to said exposedsurface of said dielectric layer;

complementary electrode means adjacent to said record medium andpositioned in spaced proximity to said charging electrode means; firstvoltage means electrically connected to said charging electrode meansfor applying thereto a first voltage of given polarity and of a firstpredetermined amplitude;

second voltage means electrically connected to said complementaryelectrode means for applying thereto a voltage pulse of polarityopposite to said given polarity and of a second predetermined amplitude,said voltage pulse having a duration sufficiently long to change bycapacitive coupling and ohmic connection the potential of saidconductive layer under said charging electrode means, said change ofpotential in coincidence with the presence of said first voltage at saidcharging electrode means causing an electrostatic latent image of theend face means said charging electrode means to be formed on said mediumat a location opposite said charging electrode means, said first voltagealone and said voltage pulse on said complementary electrode means alonebeing insufficient to cause an electrostatic latent image to be formed.

2. In an electrographic imaging system comprising a record medium havinga conductive layer in contact with a dielectric layer with an exposedsurface, and an electrostatic charging means including:

charging electrode means having end face means adjacent to saiddielectric layer;

complementary electrode means having portion means adjacent to saidrecord medium and positioned in spaced proximity to said chargingelectrode means; first voltage means electrically connected to saidcharging electrode means for applying thereto a first voltage of givenpolarity and of a first predetermined amplitude;

second voltage means electrically connected to said complementaryelectrode means for applying thereto a voltage pulse of polarityopposite to said given polarity and of a second predetermined amplitude;

said voltage pulse producing a change by capacitive coupling of thevoltage of that portion of the conductive layer directly opposite saidcomplementary electrode means, and said change producing by ohmicconnection a change in the potential of a portion of the conductivelayer under said charging electrode means, said change of potential ofsaid conductive layer under said charging electrode means in coincidencewith the presence of said first voltage at said charging electrode meanscausing an electrostatic latent image of the end face means of saidcharging electrode means to be formed on said medium at a locationopposite said charging electrode means, said first voltage alone andsaid voltage pulse on said complementary electrode means alone beinginsufficient to cause an electrostatic latent image to be formed.

3. An electrostatic charging system as set forth in claim 2 means is adielectric material.

6. An electrostatic charging head for use with a relatively movingrecord medium comprising:

an array of small electrical conductors each having an end face having agiven length in the direction of motion of said record medium.

support means formed of dielectric material supporting said array ofelectrical conductors in closely spaced relation to each other with thesaid end faces exposed and substantially flush with a face of saidsupport means,

complementary electrode means having an area large compared to the totalarea of the end faces of said array of small electrical conductors, andhaving a given length in the direction of motion of said record mediumsubstantially greater than the said given length of said smallelectrical conductors.

said complementary electrode means being mounted on said support meansadjacent to but spaced from the end faces of said small electricalconductors forming with said support means and with the ends of saidsmall electrical conductors a substantially flush, smooth surface overwhich said record medium can move,

and electrical contacts mounted on said support means in electricalcontact with said array of small electrical conductors and in electricalcontact with said complementary electrode means.

7. An electrostatic charging head as set forth in claim 6, furthercharacterized by said array of small electrical conductors comprising afirst line of conductors extending in a direction transverse to thedirection of motion of said record member, and a second line ofconductors spaced closely adjacent said first line of conductors andextending parallel thereto, the individual conductors forming saidsecond line being staggered in respect to the individual conductorsforming said record line.

8. An electrostatic charging head as set forth in claim 6, furthercharacterized by the end faces of said small electrical conductors beinground.

9. An electrostatic charging head as set forth in claim 6, furthercharacterized by the end faces of said small electrical conductors beingrectangular.

10. An electrostatic charging head as set forth in claim 6, furthercharacterized by:

said array of small electrical conductors comprising a plurality ofgroups of small rectangular electrical conductor, each of said groups ofrectangular conductors defining a matrix for forming a character.

11. An electrostatic charging head as set forth in claim 6, furthercharacterized by said array of small electrical conductors extending ina line transverse to the direction of motion of said record member, andby said complementary electrode means comprising a plurality ofelectrodes at least one on either side of said array.

12. An electrostatic charging head as set forth in claim 11 furthercharacterized by said line transverse to the direction of motion of saidrecord member being at a given angle other than 90 thereto, and by saidcomplementary electrode means extending at said given angle.

2. In an electrographic imaging system comprising a record medium havinga conductive layer in contact with a dielectric layer with an exposedsurface, and an electrostatic charging means including: chargingelectrode means having end face means adjacent to said dielectric layer;complementary electrode means having portion means adjacent to saidrecord medium and positioned in spaced proximity to said chargingelectrode means; first voltage means electrically connected to saidcharging electrode means for applying thereto a first voltage of givenpolarity and of a first predetermined amplitude; second voltage meanselectrically connected to said complementary electrode means forapplying thereto a voltage pulse of polarity opposite to said givenpolarity and of a second predetermined amplitude; said voltage pulseproducing a change by capacitive coupling of the voltage of that portionof the conductive layer directly opposite said complementary electrodemeans, and said change producing by ohmic connection a change in thepotential of a portion of the conductive layer under said chargingelectrode means, said change of potential of said conductive layer undersaid charging electrode means in coincidence with the presence of saidfirst voltage at said charging electrode means causing an electrostaticlatent image of the end face means of said charging electrode means tobe formed on said medium at a location opposite said charging electrodemeans, said first voltage alone and said voltage pulse on saidcomplementary electrode means alone being insufficient to cause anelectrostatic latent image to be formed.
 3. An electrostatic chargingsystem as set forth in claim 2 wherein said complementary electrodemeans and said charging electrode are on the same side of the medium. 4.An electrostatic charging system as set forth in claim 2 wherein saidcomplementary electrode means and said charging electrode means are onopposite sides of the medium.
 5. An electrostatic charging means as setforth in claim 2 wherein at least a portion of said complementaryelectrode means is a dielectric material.
 6. An electrostatic charginghead for use with a relatively moving record medium comprising: an arrayof small electrical conductors each having an end face having a givenlength in the direction of motion of said record medium. support meansformed of dielectric material supporting said array of electricalconductors in closely spaced relation to each other with the said endfaces exposed and substantially flush with a face of said support means,complementary electrode means having an area large compared to the totalarea of the end faces of said array of small electrical conductors, andhaving a given length in the direction of motion of said record mediumsubstantially greater than the said given length of said smallelectrical conductors. said complementary electrode means being mountedon said support means adjacent to but spaced from the end faces of saidsmall electrical conductors forming with said support means and with theends of said small electrical conductors a substantially flush, smoothsurface over which said record medium can move, and electrical contactsmounted on said support means in electrical contact with said array ofsmall electrical conductors and in electrical contact with saidcomplementary electrode means.
 7. An electrostatic charging head as setforth in claim 6, further characterized by said array of smallelectrical conductors comprising a first line of conductors extending ina direction transverse to the direction of motion of saiD record member,and a second line of conductors spaced closely adjacent said first lineof conductors and extending parallel thereto, the individual conductorsforming said second line being staggered in respect to the individualconductors forming said record line.
 8. An electrostatic charging headas set forth in claim 6, further characterized by the end faces of saidsmall electrical conductors being round.
 9. An electrostatic charginghead as set forth in claim 6, further characterized by the end faces ofsaid small electrical conductors being rectangular.
 10. An electrostaticcharging head as set forth in claim 6, further characterized by: saidarray of small electrical conductors comprising a plurality of groups ofsmall rectangular electrical conductor, each of said groups ofrectangular conductors defining a matrix for forming a character.
 11. Anelectrostatic charging head as set forth in claim 6, furthercharacterized by said array of small electrical conductors extending ina line transverse to the direction of motion of said record member, andby said complementary electrode means comprising a plurality ofelectrodes at least one on either side of said array.
 12. Anelectrostatic charging head as set forth in claim 11 furthercharacterized by said line transverse to the direction of motion of saidrecord member being at a given angle other than 90* thereto, and by saidcomplementary electrode means extending at said given angle.